You used to be able to place transparent blocks above a world’s natural spawn point. Things like leaves, stairs, glass, etc. and still spawn on the first solid block. Now any block causes you to spawn above it. If you have cheats enabled you can still set spawn via the Y level, but “natural spawn points” of a world can no longer let players spawn underneath, even with plenty of room. So realms with a vanilla playstyle can not create rooms to spawn in any more.